CVE-2016-0634 in Bash
Summary
by MITRE
The expansion of '\h' in the prompt string in bash 4.3 allows remote authenticated users to execute arbitrary code via shell metacharacters placed in 'hostname' of a machine.
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Analysis
by VulDB Data Team • 12/16/2022
The vulnerability identified as CVE-2016-0634 represents a critical security flaw in the bash shell version 4.3 that enables remote authenticated attackers to execute arbitrary code through manipulation of hostname values. This vulnerability specifically targets the prompt string expansion mechanism within bash, where the '\h' escape sequence is intended to display the hostname of the machine. The flaw occurs when bash processes prompt strings containing this escape sequence and fails to properly sanitize hostname values that may contain shell metacharacters. When an attacker can influence the hostname value of a system, they can craft malicious input that includes shell metacharacters such as semicolons, pipes, or command substitutions. This vulnerability falls under the category of command injection attacks and is classified as CWE-78, which represents improper neutralization of special elements used in OS commands. The issue stems from bash's inadequate handling of escape sequences in prompt strings, creating a path for attackers to inject malicious commands that will be executed in the context of the shell process.
The operational impact of CVE-2016-0634 extends beyond simple command execution, as it can enable attackers to gain persistent access to systems and escalate privileges through various attack vectors. When an attacker successfully exploits this vulnerability, they can execute arbitrary commands with the privileges of the user running the shell, potentially leading to complete system compromise. The vulnerability is particularly concerning because it can be exploited through various authentication mechanisms, including SSH connections where hostname information is often displayed in prompts. Attackers can manipulate the hostname value through various means such as modifying system configuration files, exploiting other vulnerabilities that allow hostname changes, or through network protocols that transmit hostname information. This vulnerability is particularly dangerous in environments where users may be authenticated through SSH or other network services that display hostname information in their prompts. The attack pattern aligns with the MITRE ATT&CK framework under the technique T1059.001 for command and scripting interpreter and T1068 for exploit for privilege escalation.
Mitigation strategies for CVE-2016-0634 focus primarily on patching the affected bash version and implementing proper input validation mechanisms. The most effective solution involves upgrading to bash version 4.4 or later, where the vulnerability has been addressed through improved sanitization of hostname values in prompt strings. Organizations should implement comprehensive patch management procedures to ensure all systems running bash are updated promptly. Additionally, administrators should consider implementing network segmentation and access controls to limit the potential attack surface, as well as monitoring for unusual hostname changes that may indicate exploitation attempts. The vulnerability demonstrates the importance of proper input validation and sanitization in shell environments, particularly when dealing with user-supplied data that may be processed through shell expansion mechanisms. Security teams should also implement regular vulnerability assessments to identify systems running vulnerable bash versions and establish monitoring procedures for detecting potential exploitation attempts through unusual command execution patterns or unauthorized hostname modifications. The fix implemented by the bash developers involved strengthening the sanitization of escape sequences in prompt strings to prevent shell metacharacters from being interpreted during hostname expansion, thereby closing the command injection pathway that attackers could exploit to execute arbitrary code.